Invariably, they won't insure anyone on something 'performance'-like until you're 25.
Adrian Flux might give you a quote as one of the specialists.
Chris Knott is another, who insist on at least a year's experience of a 'performance car' before quoting you, which you have, but they generally run to a 25+ rule. Give them a call and see if there's any way around that.
If not, stick to whatever is cheapest in the Admiral group for you. It can often be the only option for those of us in the 20-25 bracket who drive something fast, and it is for me in the Z4.